This invention relates to an ion implanter used for the fabrication of semiconductor elements, and in particular to an ion implanter permitting to reduce the implantation time and to increase the number of treated wafers, when they are silicon wafers having a large diameter.
In the field of the fabrication process of semiconductor elements, the ion implantation method is utilized, by which impurity ions such as phosphorus, boron, arsenic, etc. are implanted for the formation of the emitter and the base of bipolar transistors or for the formation of the source and the drain of MOS transistors. FIG. 1 is a scheme illustrating the ion-implantation chamber of a high-current ion implanter, which is used at present in a production line (cf. "Semiconductor World", 1982, 8, p. 43). The diameter of wafers usually used at present is 4 inches (100 mm) or 5 inches (125 mm). 25 wafers forming one unit for semiconductor process line are mounted as a batch on the surface of a rotating disk 1. Uniform ion implanation of wafers is carried out by mechanically moving the disk 1 in a radial direction and by rotating it with respect to a stationary ion beam.
Now, in the fabrication process of semiconductor elements, the diameter of the wafers tends to be larger and larger and it is anticipated that wafers having a diameter of 6 to 10 inches will be utilized in the near future. When wafers having such a large diameter are mounted in a prior art ion-implanation chamber as illustrated in FIG. 1, only several wafers can be mounted. It is also conceivable to increase the diameter of the rotating disk 1 so that the number of mounted wafers is increased. However, this method results in an extremely large ion-implantation chamber and thus this improvement is not practical method. The maximum size of the disk, which can be practically applicable to a present implanter is about 1 m in diameter.
On the other hand, in the prior art device as illustrated in FIG. 1, the wafers can be exchanged, in general, only after the atmospheric pressure has been reestablished in the ion-implantation chamber. The time necessary for wafer exchange operation, including the evacuation operation, is at shortest from several minutes to about ten minutes. The number of wafers, which can be treated for those having a large diameter, is at most several tens of wafers per hour. That is, even if the ion beam current is increased, the time necessary for the wafer exchange and the evacuation operation is longer than that necessary for the ion-implanation itself. Therefore, the wafer throughput is limited with the time necessary for the wafer exchange and evacuation operation. Consequently, in order to increase the number of wafers, which can be treated, for those having a large diameter, various ion-implantation methods permitting to reduce the wafer exchange time and the evacuation operation time, are proposed. For example, a method is known, by which the ion beam is magnetically scanned in one direction by means of a magnetic field and at the same time the wafers are moved mechanically in the direction perpendicular to the magnetic scanning.
In this case the introduction of the wafers one by one into the ion-implantation chamber in vacuum is required. Mechanical apparatus which provides one wafer per every 10 seconds into vacuum chamber, have been already put to practical use. Wafer throughput of about 360 wafers per hour can be usually realized for wafers with a large diameter. In this case, the throughput depends on the intensity of the ion beam current. However, the prior art magnetic scanning method as described above has a drawback that the wasteful area, where ions are implanted wastefully due to beam over-scanning, increases with wafer diameter increase.
FIGS. 2a and 2b are schemes for explaining the prior art uniform ion-implantation method with the magnetic scanning. The ion beam 3 is magnetically swept in a horizontal direction by means of an N pole and an S pole, as indicated in FIG. 2a. Wafers 2 are moved mechanically e.g. downward. Usually the amplitude of the ion beam scan is kept constant and the velocity of the mechanical movement of the wafers 2 is also kept constant. The velocity of this mechanical movement depends on the magnitude of the ion beam current and the ion dose. However, by such a prior art method, ions were implanted also in the hatched region in FIG. 2b. In general, the area of this region is compatible with wafer area. In particular, when the wafer diameter is large, this wasteful area is also large. For example, Japanese Utility Model Unexamined Publication Nos. 57-182864 (1982) and 58-79948 (1983) can be cited as prior art techniques relating to this sort of ion implanter in which a magnetic type beam scanner is used.